1. Technical Field
The invention relates to near infrared (NIR) absorbing dyes. In particular, the invention relates to one or both phthalocyanine and naphthalocyanine dyes with extended conjugation.
2. Description of Related Art
Phthalocyanine and naphthalocyanine are macrocylic compounds that have strong pigmenting power attributed to their alternating nitrogen and carbon atom central ring structures of porphyrin compounds (i.e., benzene-porphyrin and naphthalene-porphyrin compounds, respectively). As such, phthalocyanine and naphthalocyanine are useful in dyes with a plethora of applications in biology and chemistry as well as industry. The central nitrogen atoms of a pyrroline portion of the porphyrin compounds may be either complexed with a metal or are metal-free to provide further variations in color. For example, metal-free phthalocyanine has a blue-green color while copper-complexed phthalocyanine has a blue color (Pigment Blue 15). Other substitutions on the phthalocyanine and naphthalocyanine compounds invoke other colors as well. In general, these phthalocyanine dyes exhibit absorption at wavelengths up to 800 nm.
Phthalocyanine and naphthalocyanine are chemically stable compounds that are normally not soluble in water or aqueous solutions. Water soluble groups can be added to increase the water solubility of the phthalocyanine and naphthalocyanine compounds. Phthalocyanine and naphthalocyanine compounds are often made in a single step synthetic process from their precursors such as aromatic 1,2-dicyano- or 1,2-carboxylic acids or anhydrides and their corresponding metallic compounds. To solubilize either of their structures, one or both of highly ionic groups and soluble ethylene oxide groups must be attached to dissolve them in water. Unfortunately, commercial water soluble cyanine compounds with near IR absorption have relatively poor stability in water or aqueous solution. Any degradation in solution that changes their physical or chemical nature can destroy their conjugation, such that their characteristic absorption (color) may be lost.
Hence, a phthalocyanine dye with absorption greater than 800 nm that is water soluble and stable in aqueous solution over time would satisfy a long felt need.